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Table 1 Overview of the definitions and interpretations of the causal mediation effects

From: Mediation analysis methods used in observational research: a scoping review and recommendations

Effect Definition Interpretation
CDE Yi(x, m) – Yi(x, m) The direct effect of changing the individual’s exposure value from x to x*, while holding the mediator value constant at m.
PNDE Yi(x, Mi(x)) − Yi(x, Mi(x)) The natural direct effect of changing the individual’s exposure value from x to x*, while holding the mediator for that individual constant at the value that would naturally be observed under exposure value x*.
TNDE Yi(x, Mi(x)) − Yi(x, Mi(x)) The natural direct effect of changing the individual’s exposure value from x to x*, while holding the mediator for that individual constant at the value that would naturally be observed under exposure value x.
PNIE Yi(x, Mi(x)) − Yi(x, Mi(x)) The natural indirect effect of changing the individual’s mediator value from M(x*) to M(x), while holding the individual’s exposure value constant at x*.
TNIE Yi(x, Mi(x)) − Yi(x, Mi(x)) The natural indirect effect of changing the individual’s mediator value from M(x*) to M(x), while holding the individual’s exposure value constant at x.
TE Yi(x, Mi(x)) − Yi(x, Mi(x)) The total effect of changing the individual’s exposure value from x to x*.
  1. Abbreviations: CDE, controlled direct effect; PNDE, pure natural direct effect; TNDE, total natural direct effect; PNIE, pure natural indirect effect; TNIE, total natural indirect effect; TE, total effect